This invention relates to a fuel-operated heating system, particularly to an auxiliary heater for a vehicle, that has a burner projecting into a combustion chamber formed by a combustion tube and has a heat exchanger surrounding the combustion tube in a manner creating an annular space through which combustion gases are exhausted.
A fuel-operated heating system of the initially mentioned type is known as an auxiliary heater for vehicles from U.S. Pat. No. 4,216,759. In this case, projections are arranged on the outer surface of the wall surrounding the combustion chamber which reduce direct contact between the combustion gases flowing through the annular space and the peripheral wall of the combustion chamber. As a result, the combustion gases flowing through the annular space are deflected toward the heat exchanger and heated considerably less by the peripheral wall of the combustion chamber, without significant impairment of the heat radiation from the peripheral wall of the combustion chamber to the interior wall of the heat exchanger. The narrow points that are defined by the ends of the projections in the annular space result in a locally faster flow velocity permitting a better heat transfer. As a result, in the case of the known auxiliary vehicle heater, a relatively low exhaust gas temperature and a correspondingly high efficiency are obtained.
From German Pat. No. 29 48790, a jacket-type heat radiation tube is known that has a central combustion tube and a jacket tube concentrically surrounding the combustion tube with a space therebetween. The combustion gases coming out of the mouth of the combustion tube, via the space formed between the combustion tube and the jacket tube, flow back to the base of the jacket heat radiation tube. This ring-shaped space has a cross section that varies over the length of the combustion tube and preferably decreases from the mouth of the combustion tube in the direction of the base of the jacket heat radiation tube. As a result, a uniform emission of heat for the length of the jacket is possible without difficulty because the cross section of the ring-shaped space affects the heat transmission by convection. However, in the case of this jacket heat radiation tube, no heat transfer device is provided and only a uniform heat emission is to be achieved over the length of the jacket by the combustion gases returned to the base of the jacket heat radiation tube in the ring-shaped space.
From German Offenlegungsschrift No. 30 11 249, a flame tube of an oil gasification burner is known where the combustion tube is shorter than the jacket tube, and its input diameter is larger than its output diameter. By means of this design, an exhaust-gas quantity that is required for optimal gasification with respect to the supplied heating oil and fresh air is achieved so that, despite optimal combustion, pressure losses are minimal.
In the case of the fuel-operated heating system developed as an auxiliary heater for vehicles of that type in U.S. Pat. No. 4,216,759, it was found that the heat transmission in the area of the annular space through which the combustion exhaust gases flow varies extensively, an extremely intensive transmission taking place in the area at which the combustion gases enter into the annular space. On the other hand, in the area of the combustion gas outlet, the heat transmission is only minimal so that the combustion gas temperature in the area of the combustion gas outlet, especially in the case of an increased heating capacity of the heating system, cannot be sufficiently lowered to the desired extent so that a less favorable efficiency has to be accepted. If, as previously customary, the annular space, between its inlet side and its outlet side, has a cross section that remains the same, the heat transmission becomes worse, in the direction toward the gas outlet, because the volume of the combustion gases in the annular space becomes smaller as they cool so that the pressure of the gases is lowered and there is also a decrease in the velocity of the combustion gases.
The invention, therefore, has a primary objective to further develop a fuel-operated heating system, particularly an auxiliary heater for a vehicle of the described type, in such a way that the heat exchanger receives an optimized and uniform transmission of heat, so that the heating system, in the case of a lower and higher heating capacity, operates at a higher efficiency and the combustion gas temperature in the area of the combustion gas outlet can, therefore, be lowered.
According to preferred embodiments of the invention, a fuel-operated heating system, particularly an auxiliary heater, is provided that achieves the noted object by the cross section of the annular space, through which the combustion gases are exhausted, being narrowed in an out-flow direction. Also transference of heat between the combustion chamber and gases in the annular space may be shielded in correspondence with the narrowing thereof to prevent reheating thereof.
Because in the fuel-operated heating system according to the invention the cross section of the ring-shaped space through which the combustion gas flows becomes narrower or smaller in the direction of the combustion gas outlet, it is achieved that the area of the combustion gas outlet also actively participates in the transmission of heat to the heat exchanger so that the annular space can be fully utilized over its whole length to the exhaust outlet for the transfer of heat to the heat exchanger. The reason is that, among other things, the flow velocity of the combustion gas is also important for the heat transfer and it is increased because of the feature of the annular space, according to the invention, narrowing in cross section in the direction of the combustion gas outlet corresponding to the decrease in volume caused by cooling so that in the annular space there exists an approximately constant flow velocity from the inlet side to the outlet side. This results in a significantly better heat transmission and the exhaust gas temperature can be lowered significantly. Thus, it is possible, in the case of the heating system according to the invention, to obtain a better efficiency than previously obtainable in the case of a lower and higher performance requirement. Additionally, the combustion temperature at the exhaust outlet is also lowered. Moreover, these advantages are achieved, according to the invention, in such a way that the heating system can remain dimensionally unchanged.
According to one preferred embodiment of the invention, the cross section of the ring-shaped space narrows continuously toward the exhaust outlet, preferably by providing the annular space with a conical shape. In the case of such a continuous size reduction, local increases of flow velocity are avoided since the velocity increases continuously because of the narrowing of the cross section, from the inlet side of the annular space toward its outlet side. The flow friction losses are also very low in this case.
In the case of an alternative embodiment, the cross section of the ring-shaped space narrows in steps, preferably several steps, in the flow direction of the combustion gases through the annular space.
In either case, the narrowing of the cross section is achieved either by the fact that the combustion tube delimiting the combustion chamber is shaped in a complementary fashion with respect to the narrowing of the cross section, or that a conventional cylindrical combustion tube is used to define the combustion chamber, but the combustion tube is surrounded by an appropriately contoured top part. When a top part is arranged over the exterior wall of the combustion tube to narrow the annular space, the advantage is achieved, as compared to the use of a reshaped combustion tube, that the geometry of the combustion chamber may remain unchanged so that no threat of impairment of the combustion process in the combustion chamber, as a result of changing cross section of the combustion chamber, exists.
When a top part is used, this top may form a separate component that creates a space between the top part and the combustion tube into which combustion gases also enter and which shield against the heat radiating from the combustion tube to the combustion gas passing through the annular space, especially in proximity of the exhaust outlet. By this measure, it is, on the one hand, achieved that the combustion tube and thus the combustion chamber remain hotter so that more favorable combustion values are obtained, and that, on the other hand, the combustion gases, before coming out of the exhaust outlet, are not reheated by radiating heat.
On the other hand, the top part may also be arranged on the exterior wall of the combustion tube without clearance, so as to be firmly connected with it, and in order to ensure a compact construction. It is also possible to use a single part that corresponds configurationally to the combined combustion tube and top part, preferably made of a ceramic material, which simplifies manufacture and assembly.
These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, several embodiments in accordance with the present invention.